In recent years, the development of advanced driving assistant system (ADAS) technologies has made it possible to automatically detect the presence of dangerous objects, such as pedestrians and motorcycles, which drivers of vehicles need to avoid with caution, and notify the drivers of the presence of dangerous objects by sound messages. In such a case, a driver is more alerted to a dangerous object if he or she hears an alert message from the direction in which the dangerous object is present. For example, in the case where the dangerous object is on the right side of the driver, the warning is given near the right ear of the driver. FIG. 10 is a diagram illustrating how to effectively deliver an alert message to a driver 61 in a vehicle 50. As illustrated in FIG. 10, for example in the case where a motorcycle is approaching on the right side of the vehicle 50, the sound “motorcycle is approaching from right” is produced near the right ear of the driver 61 driving in the driver's seat 51 of the vehicle 50. For example in the case where the vehicle 50 is drifting out of the left lane, the sound “drifting out of left lane” is produced near the left ear of the driver 61. To produce such sound near the left and right ears of the driver 61, a sound signal input to each of an L channel (left channel) speaker 88 and an R channel (right channel) speaker 89 included in the vehicle 50 is processed as appropriate.
Automated driving technologies have advanced in recent years, and more driving operations are likely to be automated in the future. However, fully automated driving has not been achieved yet. In the case where control by automated driving is difficult, control needs to be transferred to the driver so that the driver performs driving operation. In this case, the driver needs to be in a comfortable awake state without being sleepy. To keep the driver in a comfortable awake state, giving a message in the ear to urge the driver to be awake is effective in the case where the driver appears to be sleepy. For example, a message such as “Hey, wake up.” may be given near the driver's ear.
A technique of producing sound near the ears will be described below, with reference to FIG. 11. FIG. 11 is a block diagram schematically illustrating a process of producing sound near the ears. FIG. 11 illustrates an example of a process of simultaneously producing sound near the left and right ears of the driver 61.
As illustrated in FIG. 11, sound is generated by processing a first sound signal and a second sound signal by virtual sound image localization filters 81 and 82 and a crosstalk cancellation unit 80. The first sound signal is a signal representing sound to be produced near the left ear of the driver 61, and the second sound signal is a signal representing sound to be produced near the right ear of the driver 61.
The virtual sound image localization filter 81 is designed so that the sound of the sound signal input to the filter is heard from the left direction of the driver 61, that is, the sound of the sound signal is localized on the left side of the driver 61. In other words, the virtual sound image localization filter 81 is designed to localize the sound of the sound signal to a predetermined position so that the sound is perceived with enhancement in the position of the left ear of the driver 61 facing the L channel speaker 88 and the R channel speaker 89. Specifically, the virtual sound image localization filter 81 is a filter representing a transfer function (V1=hVL) of sound from a sound source placed in the left direction of the driver 61 to the left ear of the driver 61.
The virtual sound image localization filter 82 is designed so that the sound of the sound signal input to the filter is heard from the right direction of the driver 61, as with the virtual sound image localization filter 81. Specifically, the virtual sound image localization filter 82 is a filter representing a transfer function (V2=hVR) of sound from a sound source placed in the right direction of the driver 61 to the right ear of the driver 61.
The first sound signal processed in the virtual sound image localization filter 81 is input to one input terminal of the crosstalk cancellation unit 80. The second sound signal processed in the virtual sound image localization filter 82 is input to another input terminal of the crosstalk cancellation unit 80.
The crosstalk cancellation unit 80 is a processing unit that performs a cancellation process of preventing the sound of the sound signal to be perceived in one ear of the driver 61 from being perceived in the other ear of the driver 61, on the sound signal.
The crosstalk cancellation process includes a process of multiplication by transfer functions A, B, C, and D, a process of addition of a signal multiplied by the transfer function A and a signal multiplied by the transfer function B, and a process of addition of a signal multiplied by the transfer function C and a signal multiplied by the transfer function D. In other words, the crosstalk cancellation process is a process using the inverse of a 2×2 matrix having, as elements, transfer functions of sounds output from the L channel speaker 88 and the R channel speaker 89 and reaching the ears of the driver 61. The transfer functions A, B, C, and D are determined based on transfer functions hFL and hCL from the L channel speaker 88 to the respective left and right ears of the driver 61 and transfer functions hCR and hFR from the R channel speaker 89 to the respective left and right ears of the driver 61.
In FIG. 11, the transfer functions hFL, hCL, hFR, and hCR indicating the transfer characteristics from the L channel speaker 88 and the R channel speaker 89 to the left and right ears of the driver are functions calculated or measured beforehand. The transfer functions hVL and hVR are functions indicating the transfer characteristics of sound from virtual speakers, which are assumed to be placed at the left and right ears, to the ears. The transfer functions A, B, C, and D are functions that serve as filters for realizing the inverse of the characteristics of the acoustic space realized by the transfer functions hFL, hCL, hFR, and hCR. The transfer functions A, B, C, and D are given by the following Formula 1.
                    [                  Math          .                                          ⁢          1                ]                                                                      (                                                    A                                            B                                                                    C                                            D                                              )                =                                            (                                                                                          h                      ⁢                                                                                          ⁢                      F                      ⁢                                                                                          ⁢                      L                                                                                                  h                      ⁢                                                                                          ⁢                      C                      ⁢                                                                                          ⁢                      R                                                                                                                                  h                      ⁢                                                                                          ⁢                      C                      ⁢                                                                                          ⁢                      L                                                                                                  h                      ⁢                                                                                          ⁢                      F                      ⁢                                                                                          ⁢                      R                                                                                  )                                      -              1                                .                                    Formula        ⁢                                  ⁢        1            
The transfer functions V1 and V2 in FIG. 11 are respectively the transfer functions hVL and hVR. With such a structure, the sounds produced from the L channel speaker 88 and the R channel speaker 89 sound as if they are produced near the ears of the driver (Patent Literature (PTL) 1).
As acoustic devices in the interiors of vehicles and the like, a device in which a steering wheel is provided with speakers has been proposed (PTL 2). The device described in PTL 2 is intended to effectively offer call function, music playback function, and the like to the driver by appropriately arranging speakers, microphones, and the like at the steering wheel.